The present invention relates to a nickel-metal hydride storage battery configured with an electrode comprising a hydrogen storage alloy capable of electrochemically absorbing and desorbing hydrogen as an active material in a reversible manner and a hydrogen storage alloy for the same.
Alkaline storage batteries having a wide application in various portable appliances include nickel-cadmium storage batteries, nickel-metal hydride storage batteries and the like. Among them, the nickel-metal hydride storage batteries have recently been used widely as small-size sealed storage batteries, because they can be expected to have a higher capacity density.
For the hydrogen storage alloys which play an important role in these nickel-metal hydride storage batteries, there is a proposal of using alloys having a high capacity obtained by improving the ZrMn.sub.2 alloy system as disclosed in the specification of U.S. Pat. No. 4,946,646, Japanese Laid-Open Pat. Publication Sho 63-284758, Japanese Laid-Open Pat. Publication Hei 1102855, and the like.
However, these prior art hydrogen storage alloys remain to be improved more with respect to their capacity, their cycle life and the like before their application for configuring batteries. That is, in addition to the main alloy phase, the prior art hydrogen storage alloys contain such minor phases as segregated phase and impurity phase that do not function as an effective phase in the hydrogen storage alloy and cause a lowering of the capacity of the alloy. In general, the more homogeneous the metal material, the more excellent the material becomes with respect to its corrosion-resistant property. The prior art hydrogen storage alloys contain minute segregated or impurity phases and are not homogeneous microscopically, and thus have an insufficient corrosion-resistant property for an alkaline electrolyte. Such component of the alloy that is liable to be dissolved, for instance, Mn or the like in particular, selectively dissolves in the alkaline electrolyte, and leads to a decrease in capacity of the alloy and eventually a lowering of the cycle life of the battery configured with the alloy.